1. Field of the Invention
This invention relates to a composition and method for treating a subterranean reservoir penetrated by a well to increase the permeability of the reservoir surrounding the well. More particularly, the invention relates to a composition and method which are especially suitable for treating a hydrocarbon-producing reservoir containing silicates and/or carbonates by removing from such reservoir carbonaceous deposits, siliceous deposits and portions of the reservoir rock.
2. Description of the Prior Art
It is often desired to increase the permeability of a subterranean reservoir penetrated by a well so that a fluid can flow more easily into and out of the reservoir via the well. Many reservoirs originally have a low permeability due to the nature and configuration of the reservoir rock. Other reservoirs become plugged or partially plugged with various deposits due to the flow of fluids therethrough. The plugging material can be either organic in nature, such as carbonaceous materials deposited from liquid or gaseous petroleum, or inorganic in nature, such as alkaline earth metal carbonates deposited from an aqueous liquid or finely divided particulate solid reservoir materials such as silicates and/or carbonates which shift position, swell or otherwise changes in character when contacted by a fluid flowing through the reservoir. Previously, various compositions have been proposed for treating subterranean reservoirs to increase their permeability. Such compositions have included numerous organic solvents for organic deposits as well as various acidizing solutions for inorganic deposits and components of the reservoir rock. Acidizing is an especially widely practiced process for increasing or restoring the permeability of subterranean reservoirs to facilitate the flow therethrough of formation fluids, including oil, gas or a geothermal fluid, and injected fluids, including enhanced recovery drive fluids. Acidizing involves injecting into the reservoir an acid, typically hydrochloric acid, in order to dissolve portions of the reservoir rock or plugging or partially plugging deposits therein, thereby opening and enlarging pore throats and other flow channels and increasing the effective porosity or permeability of the reservoir.
One problem in acidizing even moderately higher temperature reservoirs encountered in petroleum-producing wells is that the acid is rapidly consumed by the reactive material immediately adjacent the borehole before the acid can penetrate any significant distance into the reservoir. Without adequate reservoir penetration, the acidizing operation can be of little value in treating the drainage area around a well. The higher the temperature of the reservoir the more pronounced this effect becomes. In view of this and other problems, the prior art acidizing methods are limited, as a practical matter, to the acidizing of reservoirs having temperatures on the order of about 200.degree. F. and less.
A still further problem in acidizing reservoirs is in directing a greater proportion of the acidizing solution into the hydrocarbon-containing portions of the reservoir and a correspondingly lesser proportion into the watercontaining portions, improving the relative permeability of the reservoir to oil, and reducing the subsequent formation of stable emulsions of water, oil and formation fines loosened during the treatment.
Various references describe acidizing or other compositions for injection into reservoirs, which compositions contain one or more of the components of the acidizing composition of this invention.
U.S. Pat. Nos. 3,481,404 and 3,548,945 to Gidley involve a method for increasing the permeability of siliceous formations by injecting therein an aqueous solution of an acid such as hydrofluoric acid. The acid solution can contain or be followed by a low molecular weight aliphatic glycol ether such as ethylene glycol monobutyl ether.
Compositions containing various amides or amide derivatives have previously been suggested for introduction into reservoirs as part of an acidizing or other treatment. U.S. Pat. No. 3,892,275 to Lybarger et al. describes a thickened aqueous well treating fluid for gravel packing, fracturing, fluid diverting, selective plugging and fluid displacement. The compositiion is an aqueous liquid containing a cellulose ether thickener, an acidifying material, and a slow reactive pH increasing material. The acidizing material may be hydrochloric acid present in sufficient quantity to decrease the solution viscosity of the system after a selected time-temperature exposure. The slow reactive pH increasing material may be a lower organic acid amide, for example formamide, dimethylformamide or acetamide, which increases the pH of the solution to neutral after an additional length of time.
U.S. Pat. No. 3,826,312 to Richardson covers a self-neutralizing well acidizing composition containing hydrochloric acid and a slow reactive pH increasing material such as an organic acid amide, for example formamide or acetamide, which increases the pH of the solution to neutral after an additional length of time.
U.S. Pat. No. 2,238,671 to Woodhouse describes a method for increasing the fluid flow from oil wells by injecting therein as an acidizing solution an aqueous solution of a readily hydrolyzable amide such as formamide, acetamide, hydroxy acetamide, methoxy acetamide, diglycol urea and isobutoxy acetamide.
U.S. Pat. No. 2,824,833 to Cardwell et al. shows a hydrochloric acid acidizing solution containing a water-soluble gum thickener, such as karaya, tragacanth and the like.
U.S. Pat. No. 3,179,171 to Beale Jr. describes a hydrochloric acid acidizing solution containing a polymer such as polyvinylpyrrolidone.
In spite of the success of some of these aforementioned treatments, the need exists for a further improved well treating composition and method for use thereof to increase the permeability of a subterranean reservoir, especially a reservoir containing siliceous and/or carbonate material.
Accordingly, it is a principal object of this invention to provide a composition and method for increasing the permeability of a subterranean reservoir by removing therefrom plugging and partially plugging deposits and/or a portion of the reservoir itself.
A further object of the invention is to provide such a composition and method for use in a higher temperature reservoir containing siliceous material and/or carbonate material, in most instances without including a conventional corrosion inhibitor.
An additional object of the invention is to provide such a composition which includes an acidizing component.
A still further object of the invention is to provide such a composition and method wherein the rate of reaction of the acidizing composition on materials in the reservoir is retarded.
Another object of the invention is to provide such a composition and method wherein the depth of penetration of the acidizing composition into the reservoir is increased.
Still another object of the invention is to provide such a composition and method wherein the acidizing composition remains in a single phase when exposed to carbonate-containing reservoirs.
Yet another object of the invention is to provide such a composition and method wherein the permeability of the reservoir to oil is improved.
Other objects, advantages and features will become apparent from the following description.